Relatively little research has been done to investigate the way postcopulatory, prezygotic mechanisms act to isolate species at the level of the gamete. This thesis uses the naturallyhybridising, externally-fertilising system of Atlantic salmon, Salmo salar, and brown trout, S. trutta, to investigate mechanisms of hybridisation through sperm-egg interactions, much of which is poorly understood. Salmon and trout experience conspecific sperm precedence during in vitro sperm competition experiments, when sperm volumes and release times are equalised. This thesis firstly aimed to explore the dynamics of gametic interactions underlying this reproductive isolation. Manipulating the sperm entry time in interspecific sperm competitions significantly influenced the observed conspecific sperm precedence. A 2 second delay to the entry of conspecific sperm did not give hybridising males first-male sperm precedence, but neither did they gain precedence with paternity being shared between males; suggesting a mechanism of selection for conspecific sperm. Selection mechanisms were investigated through in vitro sperm competitions where egg ovarian fluid type was manipulated. Results showed that conspecific ovarian fluid allowed conspecific sperm significantly higher fertilisation success when competing against heterospecific sperm, regardless of which species eggs were under competition. This is the first evidence for cryptic female choice via a reproductive fluid in an external fertiliser. The second objective of my thesis was to investigate the potential consequences of salmon-trout hybridisation for wild populations. This was achieved through comparing the early life and reproductive fitness of hybrids and pure species. Both reciprocal hybrid crosses had comparable early life fitness to pure species. Importantly however, neither reciprocal cross exceeded pure juveniles for any fitness measures. This suggests the replacement of parental species by hybrids is unlikely. Both hybrid crosses were capable of producing viable sperm and able to fertilise over 50% of both salmon and trout eggs. Neither cross gained paternity success when competing for trout eggs with conspecific males, while very low paternity was gained under sperm competition with Atlantic salmon for salmon eggs. The main threat posed by hybridisation to vulnerable salmon populations appears to come from wasted reproductive effort, through the production of reproductively unfit hybrids. The implications of this are discussed.